Tellus, 50A, 573-595
North- Atlantic Wintertime Intraseasonal Variability
and its Sensitivity to GCM Horizontal Resolution
Francisco J. Doblas-Reyes, Michel Déqué, and David B. Stephenson
Météo-France/CNRM, 42 Avenue Coriolis
31057 Toulouse Cedex, France
Francisco Valero
Dpto. Física de la Tierra, Astronomía
y Astrofísica II,
Universidad Complutense, Ciudad Universitaria, 28040
Madrid, Spain
The ability of an atmospheric general circulation model to simulate the
wintertime extratropical intraseasonal variability is analysed, and the
dependence on the horizontal resolution is discussed. The emphasis is on
the North-Atlantic/European region circulation. Three different resolutions
using the ARPEGE model are investigated: medium (T42) and high (T106) resolution,
and a conformally stretched T63 version with maximum resolution over the
Mediterranean. Circulation features are validated against ECMWF analyses.
The high resolution simulation shows both some more realistic features
than the low resolution simulation. The conformally stretched model, however,
appears to show mainly less realistic features than in both the low and
high resolution simulations. The time-mean structure of the 500 hPa geopotential
height indicates that the model is too zonal,especially at high resolution,
with a greater than normal synoptic-scale activity. The storm tracks penetrate
too eastward over Europe, are shifted southward and do not head to the
northeast at the exit of the jet. These features are related to a too strong
barotropic forcing of the mean flow by the synoptic-scale eddies on the
equatroward side of the jet, and with an underestimation of the blocking
frequency. Despite of the unrealistically strong mean flow, the high resolution
simulation has more realistic storm tracks than at lower resolution. Low-frequency
intraseasonal variance is underestimated at each resolution, mainly north
of 50ºN. The model does not show westward travelling waves nor realistic
spatial modes of variability. Furthermore, blocking frequency is underestimated
and the blocking patterns are eastward shifted, following the storm track
penetration over Europe. The systematic errors in the simulated variability
have been successfully isolated and identified using space-time spectral
and complex principal component analyses.